1. Field of the Invention
The present invention relates to an image processing apparatus having a function of increasing picture quality, and to a circuit for detecting the contour of the waveform of a picture signal input to an image display apparatus such as a liquid crystal device or a cathode-ray tube.
2. Description of the Related Art
Conventional image display apparatuses that treat a picture signal perform a steep-contour generating process in which, when an input picture signal has insufficient high-frequency-range characteristics, a high frequency component, extracted from the input picture signal, is added to the original picture signal, and the picture signal to which the extracted high frequency component is added is processed so that the rise and fall of its contour is steep.
This conventional steep-contour generating process is described below with reference to the circuit shown in FIG. 10.
The circuit in FIG. 10 includes a band-pass filter (BPF) 1102 which extracts and outputs a high frequency component (contour component) of a video signal (luminance signal) 1101, a coring 1103 for outputting a processed signal in accordance with the level of the high frequency component extracted by the BPF 1102, and an adder 1104 for outputting the sum of the original video signal 1101 and the processed signal input from the coring 1103.
The waveforms of the signals 1101, 1105, and 1106 are shown in FIGS. 11A, 11B, and 11C, respectively.
Referring to Fig. 11C, the waveform of the sum signal 1106 is obtained by identically adding the processed signal 1105 from the coring 1103 to all the data of the original video signal 1101, regardless of the positions of waveform edges of the input video signal 1101. Thus, in the waveform of the sum signal 1106, an undershoot occurs at a portion A, and an overshoot occurs at a portion B. Therefore, the sum signal 1106 generates an image looking glaring, and may cause deterioration in the image quality.
Accordingly, it is an object of the present invention to provide a contour detecting circuit for detecting the contour of the waveform of an input video signal.
It is another object of the present invention to provide an image display apparatus for performing preferable contour correction without causing deterioration in image quality by eliminating an undershoot and an overshoot in a waveform obtained by a steep-contour generating process.
To these ends, according to an aspect of the present invention, the foregoing objects are achieved through provision of an image display apparatus including a second differentiating circuit for computing a second-differentiated value based on an input video signal, a determining circuit for detecting timing with which the polarity of the second-differentiated value obtained by the second differentiating circuit is inverted, and a computing circuit for performing a steep-contour generating process on an input video signal corresponding to the timing detected by the determining circuit so that the contour of the input video signal is steep.
According to another aspect of the present invention, the foregoing objects are achieved through provision of an image display apparatus including a first differentiating circuit for computing a first-differentiated value based on an input video signal, a second differentiating circuit for computing a second-differentiated value based on the input video signal, a detection-computing circuit for obtaining the degree of a change in the input video signal with predetermined timing from a first-differentiated value based on an input video signal with the predetermined timing and a first-differentiated value based on at least one of input video signals adjacent to the input video signal with the predetermined timing, a determining circuit for detecting timing with which the polarity of the second-differentiated value in the second differentiating circuit is inverted, and for determining, by referring to the computation result by the detection-computing circuit, which corresponds to the predetermined timing, a contour of the input video signal to be processed by a steep-contour generating process, and a computing circuit for performing, based on the determination result by the determining circuit, the steep-contour generating process on the input video signal.
According to a further aspect of the present invention, the foregoing objects are achieved through provision of a contour detecting circuit including a first differentiating circuit for computing a first-differentiated value based on an input video signal, a second differentiating circuit for computing a second-differentiated value based on the input video signal, a detection-computing circuit for obtaining the degree of a change in the input video signal with predetermined timing from a first-differentiated value based on the input video signal with the predetermined timing and a first-differentiated value based on at least one of input video signals adjacent to the input video signal with the predetermined timing, and a determining circuit for detecting timing with which the polarity of the second-differentiated value by the second differentiating circuit is inverted, and for determining, by referring to the computation result by the detection-computing circuit, which corresponds to the predetermined timing, a contour of the input video signal.
According to the present invention, the degree of a change in an input video signal can be found without being affected by an amplitude component of the input video signal. This is preferable in that only a change in the desired frequency component is found.
According to the present invention, the structure of a detection-computing circuit can be simplified.
According to the present invention, an image obtained by performing a steep-contour generating process can be adapted for a change in adjacent video signals, whereby the image is smoothed.
According to the present invention, a steep-contour generating process in a region which requires no contour correction is prevented from being performed. This enables preferable contour correction without causing deterioration in image quality by eliminating an undershoot and an overshoot in a waveform obtained by the steep-contour generating process, so that the processed image can be natural.